1. Field of the Invention
The present invention relates to a semiconductor device and a method for writing data into the semiconductor device. More particularly, the present invention relates to a semiconductor device having multi-bit memory cells and a method for writing data into the multi-bit memory cells.
2. Description of the Related Art
Recently, semiconductor memory devices such as flash memories have been demanded to have an increased memory capacity. A multi-bit semiconductor memory device has been proposed to meet the above demand. This kind of semiconductor memory devices is proposed in Japanese Patent Application Publication Nos. 8-235886 and 2002-216485. These publications show that two bits or more can be stored in a memory cell.
In the semiconductor memory device with multi-bit memory cells, 2-bit information can be stored in a memory cell with four levels described as 1, 2, 3 and 4. Each of the four levels corresponds to a respective one of four combinations of two bits of input or output data. The levels 1, 2, 3 and 4 are respectively defined as 2-bit input or output data (1, 1), (0, 1), (1, 0) and (0, 0). Here, data “0” is defined as a programmed state, and data “1” is defined as an erased state. Programming of the memory cell changes the data stored therein from “1” to “0”, and erasing changes the data from “0” to “1”. Generally, the flash memories are inhibited from changing the data “0” in the memory cell to data “1” by a write command.
When the flash memory is supplied with a write command for changing the threshold voltage Vth of the memory cell that is in the state of level-2 to level 3, the actual programming operation changes data in the memory cell from (0, 1) to (1, 0). That is, one of the two bits changes from data “0” to data “1”. This means that erasing is performed in the programming.